The development of the information industry requires technology for transmitting various kinds of large amounts of data at high speeds. To this end, a distributed antenna system (DAS) for removing a shadow region and extending coverage by providing a plurality of distributed antennas in an existing cell is being researched. A DAS is a system employing a plurality of distributed antennas connected to a single base station (BS) by wire or through a dedicated line, and the BS manages the plurality of distributed antennas which are a predetermined distance or more from each other in the cell served by the BS itself.
A DAS is distinguished from a centralized antenna system (CAS) in which a plurality of BS antennas are concentrated at the center of a cell in that a plurality of antennas are located a predetermined distance or more from each other in a cell. Also, a DAS is distinguished from a femto cell in that the unit of each distributed antenna does not have control over the region of the antenna but a BS at the center of a cell has control over all distributed antenna regions located in the cell.
Further, a DAS is distinguished from a multi-hop relay system in which a BS and a remote station (RS) are wirelessly connected or an ad-hoc network in that distributed antenna units (referred to as “RUs” below) are connected by wire or through a dedicated line. Moreover, a DAS is distinguished from a repeater structure for simply amplifying and transmitting a signal in that each distributed antenna may transmit different signals to respective terminals adjacent to the antenna according to control of a BS.
In such a DAS, RUs may support a single or multiple mobile terminals by simultaneously transmitting/receiving different data streams, and thus a DAS may be regarded as a multiple input multiple output (MIMO) system.
In a DAS, while a downlink signal is copied to each RU and then transmitted through the antenna of the RU, uplink signals are received through the antennas of all RUs and then combined and processed at a digital unit (DU). Therefore, the noise level of an uplink signal from one arbitrary RU affects the noise levels of uplink signals of all RUs.
Meanwhile, control of a noise level in a mobile communication system is intended to correct a noise level which changes along with a change of the gain of an input circuit over time. An uplink signal in a DAS is the sum of uplink signals received from all connected RUs. Therefore, when all the RUs have the same noise level, the noise level of an uplink signal finally combined at a DU increases in proportion to the number of connected RUs. On the other hand, when the noise levels of respective RUs are different, a noise level combined at a DU depends on an uplink signal of an RU having a relatively high noise level. In this case, the overall noise level is estimated to be higher than when all RUs have the same noise level, resulting in a relative reduction in cell coverage.
Therefore, it is necessary to optimize cell coverage by equally adjusting noise levels at all RUs, and for this sake, it is necessary to measure a pure noise level when there is no user traffic. To this end, Global Positioning System (GPS) equipment for measuring an absolute time may be installed in each RU, or a DU may frequently notify each RU of absolute time information. However, such a method complicates a system or a procedure and also increases cost.
This work was supported by the ICT R&D program of MSIP/KEIT, Republic of Korea. [10041628], Industrial Core Technology Development Project for next generation communication network]